Rigid-disk files consist of a stack of circular disks having a thin magnetic coating rotating at high speed. Data is recorded on the disk surfaces using heads or transducers mounted on actuator arms that are moved across the disk surfaces by a high-speed actuator. The actual information is recorded in circumferential tracks on the disk surfaces. Reading the recorded information back involves sensing the magnetic transitions emanating from the surface of the rigid-disk, again using the transducers.
The ability to store and read back information from the disk in the form of magnetic transitions may be impaired by the presence of disk defects. Disk defects can manifest themselves in several ways. One way is simply as noise associated with the random nature of the disk surface. Often, this noise can be easily separated from the ambient noise in the read back channel by measuring the noise from the preamplifier with the head flying over the disk and then repeating the measurement with the head unloaded from the disk.
More troublesome are flaws or defects in the disk surface that manifest themselves as missing bits or extra bits of data (i.e., single bit errors). Missing bits are reductions in the amplitude of the envelope of the signal, usually over a small number of bits (e.g., 1-4 bits), such that the amplitude falls below the channel-detector clipping level. The number of missing bits observed depends on the setting of the clipping level in the read back channel. One common technique for avoiding the problem of missing or extra bits is to perform a surface analysis of the rigid disk. This surface analysis yields an error map of the rigid-disk surface. The error map can then be referred to as a means of avoiding localized disk flaws.
During surface analysis the completed file is formatted and defects are located. These defect locations are stored in header fields at the start of the data records. The header fields are then referred to during reading/writing of information to the magnetic disk so that identical defect locations on the disk can be avoided. Obviously, all of this depends on having the capability of detecting the presence of a random disk defect or flaw. Over the years, various techniques for detecting disk defects have been developed.
In many modern disk-drive recording systems, the playback waveform at the highest analog frequency data pattern is a sinusoid waveform. According to one common approach, media defects are detected by writing this high frequency data pattern and then looking for deviations from the expected sinusoid upon playback. Since the expected signal is a sinusoid, a very narrow band tracking notch filter has been traditionally employed to effectively exclude the sinusoid and pass the deviations.
In the frequency domain these deviations correspond to phase and/or amplitude modulation sidebands about the carrier. While prior art systems have been generally capable of amplitude error detection, unfortunately they have failed to provide effective phase error detection. This means that disk flaws which generate phase rather than amplitude deviations almost always go undetected. Moreover, the error resolution of past systems was generally no better than the sector or byte level. In other words, smaller errors (having a resolution of a bit location) routinely pass undetected.
A further disadvantage characteristic of prior art approaches to media flaw detection is the relatively high expense and complexity of circuit components required. Very often, modern flaw detection circuitry demands external gain control and phase-lock-loop (PLL) circuitry which is not readily integrated into the disk drive system.
What is needed then is an effective, simple and inexpensive defect detection system that may be used to reliably identify the locations of flaws in a magnetic recording media. As will be seen, the present invention provides an improved apparatus and method for detecting flaws that is easily integrated into a disk drive system, thus enabling the system to create its own error map without the usual large capital equipment investment in the manufacturing operation. Furthermore, the invention is readily integrated into existing recording channel devices at very little incremental cost.
Alternatively, the circuit components may be packaged as a small test module to be temporarily plugged into the drive during the flaw mapping process as part of the disk drive manufacturing process.
In either embodiment, the flaw detector of the present invention is capable of detecting flaws down to the single bit level. In addition, its performance is independent of the flaw position relative to the recorded magnetic transitions. Due to this phase independence, a much more reliable flaw map can be formed in a single pass.